Post : Measuring a pulse with infrared light
URL : http://hackaday.com/2012/09/13/measuring-a-pulse-with-infrared-light/
Posted : September 13, 2012 at 8:01 am
Author : Brian Benchoff
Tags : heart rate, photoplethysmographic, pulse, pulse meter
Categories : Medical hacks

http://hackadaycom.files.wordpress.com/2012/09/finglonger.jpg

The next time you're unfortunate enough to make your way to a hospital, emergency room, or urgent care clinic, you'll be asked to attach a small pulse monitor to your finger. The device the nurses clip on to one of your remaining digits is called a photoplethysmographic sensor, and basically it is able to read your pulse through reflected light. In the search to find out how these devices actually work, [Raj] sent in a great tutorial (http://embedded-lab.com/blog/?p=5508) covering the theory behind photoplethysmographicy, and also built a simple device to detect a pulse without using a microcontroller.

These photoplethysmographic sensors operate by shining light into someone's flesh - usually a finger or ear lobe - and recording the light reflected back to the source. The volume of blood in the finger will have an effect on the amount of light reflected back, and makes for a perfect way to automatically measure someone's heart rate.

To build his device, [Raj] used a TCRT1000 reflective optical sensor (http://www.vishay.com/optical-sensors/list/product-83752/) . Inside this sensor is an infrared LED and a phototransistor. Of course with a finger over the sensor there is a ton of noise both from ambient light and the base rate of reflected light from a piece of flesh. [Raj] filtered this out, leaving only the small variations in the amount of reflected light, thus creating a very simple - and very inexpensive - electronic pulse meter.

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